1. Field of the Invention
The present invention relates to a driving circuit of a display and a display device and more particularly to the driving circuit of the display made up of light emitting elements including an EL (Electroluminescence) element, an LED (Light Emitting Diode) element, a VFD (Vacuum Fluorescent Display) (FED (Field Emission Display) being one of the VFDs in particular) element or a like and used to display various kinds of information, measurement results, moving pictures or still pictures and to the display device equipped with the driving circuit of the display described above.
The present application claims priority of Japanese Patent Application No. 2000-259984 filed on Aug. 29, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, some displays are made up of light emitting elements which include an EL element, LED, VFD (FED in particular), or the like. Of them, an EL display constructed of the EL elements is considered to be promising since it has many advantages in that it can be made planar, thinner, and more lightweight, that it can provide excellent visibility by spontaneous light and can provide quick response and that it can display moving pictures. Conventionally, an inorganic EL element using inorganic materials such as ZnS:Mn (zinc sulfide:manganese) or a like is mainstream, however, an organic EL element using organic materials such as a stilbene derivative or a like has been developed recently.
FIG. 9 is a schematic perspective view for showing configurations of a conventional organic EL display made up of such the organic EL element. An organic EL display 1 shown in FIG. 9 configured so as to display full color includes a plurality of data electrodes (anodes) 3 formed on a transparent substrate 2 at specified intervals and in a stripe form, a hole injected layer 4 formed on the transparent substrate 2 and on an entire surface of the data electrodes 3, a hole transporting layer 5 formed on an entire surface of the hole injected layer 4, light emitting layers 6 to 8 each emitting a green color (G) light, red color (R) light, and blue color (B) light, respectively, arranged in order of a green color light emitting element, red color light emitting element and blue color light emitting element, in a sequentially repeated manner and in a manner to correspond to column-directional arrangement of the data electrodes 3 and arranged in a manner that the light emitting layers 6 to 8 to emit light of same color out of the light emitting layers 6 to 8 are placed consecutively in a row direction, an electron transporting layer 9 formed on entire surfaces of the hole transporting layer 5 and light emitting layers 6 to 8 and a plurality of scanning electrodes (cathode) 10 formed on the electron transporting layer 9 in a row direction at specified intervals. The transparent substrate 2 is made up of glass or a like. The data electrode 3 is made up of a transparent electrode such as ITO (Indium Tin Oxide) or a like. The hole injected layer 4 and hole transporting layer 5 are made up of a triphenyldiamine derivative, carbazole derivative or the like. The light emitting layers 6 to 8 are made up of the stilbene derivative or a like. The electron transporting layer 9 is made up of a perylene derivative. The scanning electrode 10 is made up of a metal electrode such as an aluminum film. In the above organic EL display 1, each of its regions producing the green, red, and blue colors respectively is hereinafter called an organic EL element ELG, organic EL element ELR, and organic EL element ELB, respectively.
In the organic EL display 1 of this example, one pixel is made up of dot pixel portions consisting of the three organic EL elements ELG, ELR, and ELB each emitting one color out of three primary colors including green, red, and blue colors. The organic EL display 1 is called a “stripe” organic EL display since the organic EL element ELG, ELR and ELB each corresponding to each of the dot pixel portions are arranged in order of the green color light emitting EL element ELG, red color light emitting EL element ELR, and blue color light emitting ELB in a column direction and in a sequentially repeated manner and the organic EL elements to emit light of same color, out of the organic EL element ELG, ELR, and ELB are consecutively arranged in a row direction. Moreover, in the organic EL display 1 of the example, a pixel portion made up of the dot pixel portions is placed at an intersection of each of the data electrodes 3 formed at specified intervals in a column direction and each of the scanning electrodes 10 formed at specified intervals in a row direction, that is, the pixel portions made up of the dot pixel portions are arranged in a matrix form, and a character, image, or a like are displayed by light-emitting of the light emitting layers 6 to 8 corresponding to an arbitrary dot pixel portion occurring when a data signal produced based on a video signal is applied to the data electrodes 3 and a scanning signal produced based on a horizontal sync signal and a vertical sync signal is applied to the scanning electrodes 10. Therefore, the above organic EL display 1 is called a “simple-matrix EL display”.
FIG. 10 is a schematic block diagram showing an example of configurations of a conventional driving circuit to drive the organic EL display 1 having configurations described above. As shown in FIG. 10, each of the scanning electrodes 10 is installed from a right end toward a left end in a display region 1a and is routed from the left end to an outside of the display region 1a and is further connected to each of scanning terminals (not shown) mounted in the left end of the organic EL display 1 at specified intervals. The data electrodes 3 are divided into two portions at an approximately central place of the display region 1a. Each of the divided data electrodes 3 installed from the approximately central place to an upper end of the display region 1a is routed to an upper portion on an upper side of the display region 1a and is connected to each of data terminals (not shown) mounted at an upper end of the organic EL display at specified intervals. Each of the divided data electrodes 3 installed from the approximately central place to a lower end of the display region 1a is routed to a lower portion on a lower side of the display region 1a and is connected to each of data terminals mounted (not shown) at a lower end of the organic EL display 1 at specified intervals. Data signal fed from both the data terminals (not shown) existing in the upper and lower direction is applied to two data electrodes 3 existing on a same column. The above method for applying the data signal to the data electrodes 3 is called a “double scanning method”. This double scanning method is employed recently since there is a need to reduce a peak current which flows through the organic EL display 1 at a time of driving the organic EL display 1 because an IC (integrated circuit) making up data electrode driving circuits 12 and 13 described later that can withstand a high voltage is not available and since it is difficult to drive all organic EL elements only by one data electrode driving circuit due to an increase in numbers of the organic EL elements to be driven by one data electrode which has occurred to meet the demand for a larger screen and higher resolution in the organic EL display 1 and since there is an increasing demand for higher luminance in the organic EL display.
The conventional driving circuit chiefly includes a controller 11, data electrode driving circuits 12 and 13 and a scanning electrode driving circuit 14. The controller 11 produces a green video signal SG, red video signal SR, and blue video signal SB based on a video signal SP supplied from outside and feeds them to the data electrode driving circuits 12 and 13 and further produces a horizontal scanning pulse PH and vertical scanning pulse PV based on a horizontal sync signal SH and vertical sync signal SV and feeds the horizontal scanning pulse PH to the data electrode driving circuits 12 and 13 and feeds the vertical scanning pulse PV to the scanning electrode driving circuit 14. Each of the data electrode driving circuits 12 and 13 has driving sections 15 in numbers being equivalent to the number of the data electrodes 3 and produces a data green signal IDG, data red signal IDR, and data blue signal IDB each having a specified current value, respectively, using the green video signal SG, red video signal SR, and blue video signal SB, all of which are voltage signals, with the timing when the horizontal scanning pulse PH is fed from the controller 11 and then feeds each of them to each of corresponding data electrodes 3 in the organic EL display 1. The scanning electrode driving circuit 14, with the timing when the vertical scanning pulse PV is supplied from the controller 11, sequentially switches the scanning electrodes 10 in the organic EL display 1 for scanning.
The organic EL display 1 that can display full color described above is one that has been developed recently and EL displays that have become generally and commercially practical are organic EL displays made up of organic EL elements that can display a yellowish-orange monochrome color. Therefore, as ICs making up the data electrode driving circuit adapted to drive the EL display, only the ICs that have been prepared for the use in the organic EL display adapted to display the monochrome color and that are provided with driving sections having same current driving capability, are distributed commercially. It is a current status quo that such ICs as have been prepared for the use in the organic EL display adapted to display the monochrome color are also used in the data electrode driving circuits 12 and 13.
However, in the conventional organic EL display 1 that can display full color, as shown in FIG. 11 and FIG. 12, there are differences in electric characteristics among organic EL elements ELG, ELR, and ELB which are caused by differences in types of organic materials used in the light emitting layers 6 to 8 each emitting, respectively, green color light, red color light, and blue color light. FIG. 11 shows an example of applied voltage-luminance characteristics of the conventional organic EL display 1. FIG. 12 shows an example of applied voltage-current density characteristics of the conventional organic EL display 1. In FIG. 11 and FIG. 12, a curve “a” shows the characteristic of the organic EL element ELG that emits the green color light, the curve “b” shows the characteristic of the organic EL element ELR that emits the red color light and the curve “c” shows the characteristic of the organic EL element ELB that emits the blue color light. As is apparent from FIGS. 11 and 12, the characteristic of the organic EL element ELG that emits the green color light is comparatively similar to that of the organic EL element ELB that emits the blue color light. However, the characteristic of the organic EL element ELR that emits the red color light is greatly different from those of the organic EL elements ELG and ELB that emit the green color and blue color light, respectively.
For example, as shown in FIG. 11, to have each of the organic EL elements ELG, ELR, and ELB emit color light at luminance of about 10,000 (cd/m2), though applied voltage of about 7.5 (V) and about 11.2 (V) are required to emit the green color light and blue color light respectively, the applied voltage of as high as about 14.5 (V) is required to emit the red color light. If the data electrode driving circuit is constructed of ICs, it is almost impossible to set the applied voltage individually for each color and, in ordinary cases, the applied voltage is set at 12 V to 13 V using, as a reference, the case in which the red color light is emitted by the organic EL element ELR having a worst characteristic. As shown in FIG. 11, in the case of the applied voltage being 12 V, the luminance of the red color light is about 2,800 (cd/m2) while the luminance of the blue color light is about 12,000 (cd/m2) and the luminance of the green color light is as high as about 50,000 (cd/m2). As shown in FIG. 12, in the case of the applied voltage being 12 V, the current density of the green color light is about 430 (mA/cm2) and the current density of the blue color light is about 260 (mA/cm2) while the current density of the red color light is as small as about 50 (mA/cm2).
Therefore, there is shortcoming in the conventional EL display in that, if the ICs that have been prepared for the use in the organic EL display adapted to display the monochrome color and for the use in data electrode driving circuits equipped with driving sections each having the same current driving capability, are used in the above data electrode driving circuits 12 and 13, in the case of emitting the red color light, sufficient luminance cannot be attained and, in the case of emitting the blue or green color light, excessive applied voltages are fed thus causing increased power consumption. This also presents problems in that a satisfactory display of full color cannot be achieved, recent demands for high definition in the display cannot be satisfied and lowering of power consumption cannot be implemented. Moreover, in recent years, strong demands for the large screen of the display have grown and, to make large a screen of the organic EL display, a double scanning method is the essential driving method. However, even when the scanning method is employed, if the number of the organic EL elements to be driven by one driving section to achieve the large screen increases, the satisfactory display of full color is made further difficult.
There is a risk that a same inconvenience as described above occurs not only in the above organic EL display adapted to display full color but also in the full color display device made up of other light emitting elements including the LED, VFD (in FED being one type of the VFD in particular) if there are the differences in characteristics of the light emitting elements to emit light of each of the green, red, and blue colors, in the applied voltage-current density characteristics, in particular.